I'm so sorry your SIL is having such a hard time!
The first few weeks are definitely the hardest. Remind her that breast feeding is a learning process for both her and the baby - it's an entirely new experience for both of them, so it will take time to get everything running smoothly.
I wanted to give up too, but I struggled with it and now I'm so glad I did. For me, all the kinks seemed to work themselves out around 10 weeks.
If she's in that much pain, there could be a latch problem. I would contact a lactation consultant or LLL ASAP.
http://www.lalecheleague.org/WebIndex.htmlWhat helped me was setting small goals. I told myself "If I can just get through this week, then I can quit". When I made it to the end of the week, I'd tell myself again "Just bf for 3 more days, then you can quit". Eventually things get better and it can become a very enjoyable experience.
http://www.breastfeeding.com is a great site that also has a lot of info.